This invention relates to a method for the preparation of siloxane-containing bischloroformates. More particularly the method relates to a continuous method for the preparation of siloxane-containing bischloroformates in a flow reactor.
Silicone-containing copolycarbonates are prized for their unique combination of ductility, toughness, and flame retardancy. Silicone copolycarbonates are typically prepared by reaction of a mixture of a siloxane-containing bisphenol and a bisphenol such as bisphenol A under interfacial conditions with phosgene and an aqueous acid acceptor such as sodium hydroxide in water. Alternatively, silicone copolycarbonates may be prepared by reaction of a chloroformate-terminated polycarbonate oligomer with a siloxane-containing bisphenol. Typically, the reaction between the chloroformate-terminated polycarbonate oligomer and the siloxane-containing bisphenol is carried out under interfacial conditions similar to those employed when a bisphenol and a siloxane-containing bisphenol are copolymerized directly with phosgene. Such approaches to silicone-containing copolycarbonates are illustrated in Japanese Patent Application JP 9265663, European Patent Application EP 500131, U.S. Pat. No. 5,530,083, U.S. Pat. No. 5,502,134, and copending U.S. patent application Ser. No. 09/613,040.
Siloxane-containing bischloroformates are potentially attractive chemical intermediates for the preparation of silicone-containing materials, including silicone-containing copolycarbonates in which the silicone-containing monomer is incorporated into the polymer as an electrophilic species. As such, improved methods for the preparation of siloxane-containing bischloroformates represent attractive goals. The present invention provides a simple, continuous, high yield method for the preparation of high purity siloxane-containing bischloroformates which is superior to known methods of bischloroformate preparation.
In one aspect, the present invention provides a continuous method for the preparation of bischloroformates of siloxane bisphenols, said method comprising introducing into a flow reactor at least one siloxane bisphenol, at least one alkali metal hydroxide or alkaline earth metal hydroxide, and phosgene, said phosgene being introduced at a rate such that the ratio of phosgene to siloxane bisphenol OH groups is in a range between about 2.5 and about 6 moles of phosgene per mole of siloxane bisphenol OH group, said alkali metal hydroxide or alkaline earth metal hydroxide being introduced as an aqueous solution, said aqueous solution having a concentration of at least about 5 percent by weight metal hydroxide, said metal hydroxide being introduced at a rate such that the molar ratio of metal hydroxide to phosgene is in a range between about 3.5 and about 6.
In another aspect, the present invention relates to the high purity siloxane bischloroformates which may be produced by the method of the present invention.